JP2678463B2 - Refractive index measurement method - Google Patents

Description

TECHNICAL FIELD The present invention relates to a refractive index measuring method, and more particularly to a refractive index measuring method suitable for measuring a refractive index and a refractive index distribution of a plastic lens.

[Prior Art] Plastic lenses reduce the weight and cost of lenses.
Or, due to the need for aspherical lenses, etc., they have been widely used in recent years.

However, from the viewpoint of stability of the physical properties of the plastic lens, there is a drawback in that the refractive index and its distribution are unstable and the fluctuation thereof is large in manufacturing, as compared with the glass lens. Therefore, it is necessary to measure the refractive index and its distribution of each plastic lens one by one after molding. In this case, the lens cannot be destroyed, so the measurement is not easy. Especially if it is an aspherical lens.

Conventionally, there have been the following methods for measuring the refractive index of such a plastic lens.

The device used as a whole is a Mach-Schender interferometer, which uses one light beam as the reference light of the plane wave and matches the lens under test in the other light beam with a refractive index almost equal to that of the lens under test. Set by immersing in liquid. Further, as a reference for the refractive index, a glass sample having a refractive index close to the lens to be inspected and having a known refractive index is simultaneously immersed in the liquid and set. In observing the interference fringes generated by placing such an immersion device in one beam of the Mach-Schender interferometer,
The difference in the thickness of the sample was measured while the interference fringes of the book were observed, and the refractive index of the lens under test was determined from the measured value.

[Problems to be Solved by the Invention] However, the refractive index of the matching liquid fluctuates with changes in temperature and the like. Therefore, in the above-described measuring method, if the temperature changes, the range in which N interference fringes occur also changes, and the thickness of the sample in that portion must be measured each time. Further, if the refractive index distribution of the lens is not uniform, irregularly shaped interference fringes are generated, and it is difficult to interpret N fringes. Therefore, in the measurement of the refractive index in such a case, there is a drawback that quantitative interpretation is difficult. In addition, it is difficult to automate the measurement.

The present invention eliminates such conventional drawbacks, can measure the refractive index and the refractive index distribution of a test object such as a lens without destroying it, and is easy to quantitatively interpret, and automates the measurement. It is an object of the present invention to provide a method of measuring a refractive index which is also easy.

[Means for Solving the Problems] In order to achieve the above object, the refractive index measuring method of the present invention comprises a sample having a known refractive index and a shape, and an object having an unknown refractive index and a known shape. Is dipped in a matching liquid whose refractive index is slightly different from that of the test object, transmits coherent light to these, and generates an interference fringe by superimposing the transmitted light wave with a reference light wave, and the light wave transmitted through the sample. The intensity distribution of the interference fringes generated by the above is output, the refractive index of the matching liquid is obtained from the output, and the intensity distribution of the interference fringes generated by the light wave transmitted through the test object is output, and the interference is output from the output. The defocus term and the aberration term of the polynomial representing the fringes are separated, the average refractive index of the test object is obtained from the defocus term and the refractive index of the matching liquid, and the test item is calculated from the aberration term. Refractive index distribution Is characterized in that

[Operation] As illustrated in FIG. 11 and FIG. 12, the refractive index n _m is the specimen soaked in approximately equal matching liquid with a refractive index n _t of the test object was transmitted through the plane wave having a wavelength λ Consider the interference fringe W ₀ (x, y) obtained by the transmitted wave and the reference wave.

The shape of the object to be inspected is known (designed value or measured value is known), and its amount is I surface: Z ₁ = S ₁ (x, y) II surface: Z ₂ = S ₂ (x, y), If the central wall thickness is d, the total amount of Sag on both sides is Sag (x,
y) is Sag (x, y) ＝ S ₁ (x, y) ＋ S ₂ (x, y)… (1) On the other hand, the refractive index n _t (x, y) is calculated as the average refractive index n _t0 and the refractive index distribution. When separated into Δn _t (x, y), W ₀ (x, y) = Sag (x, y) · [n _t0 −n _m ] + [d−Sag (x,
_{y)] Δn t (x,} y) ... (2) (2) by measuring the refractive index n _m of the matching liquid from the equation,
W ₀ (x, y) is a function of only n _t0 and Δn _t (x, y),
Generally, these two parameters cannot be obtained separately.

Now, when the above formula (2) is expanded by using a general Zernike polynomial in the expansion formula of the wavefront aberration, W ₀ (x, y) = Sag (x, y) · [n _t0 −n _m ] + [d−Sag (x,
y)] Δn _t (x, y) = C ₁ + C ₂ ρ cos + C ₃ ρ sin + C ₄ (2ρ ² -1) + C ₅ ρ ²
You can write cos2 + ...

C ₂ ρcos and C ₃ ρsin are tilt terms, C ₄ (2ρ ² −1) is a defocus term (W ₂ ), C ₅ ρ ² cos ² + ... Is an aberration term (W).

Then, since [n _t0 −n _m ] ≈ 0, Sag (x, y) · [n
_t0 −n _m ] can be approximated by a quadratic function, and is approximately equal to the defocus term.

FIG. 13 shows that one surface is a spherical surface (curvature radius r), the other surface is a flat surface (curvature radius ∞), the diameter is D, and there are various R numbers (R = r / D) with no refractive index distribution. Consider an object to be inspected, and the object to be inspected is 3 stripes (W ₀ = 3λ) and 1 stripe (W ₀ = 1)
The value of the aberration term (ΔW) is shown on the assumption that the matching liquid is prepared so that the fringes λ) can be observed. If the reproducibility of fringe reading is λ / 25 in Rms, R> 0.7 even when the number of fringes is three.

Therefore, most of the R-number test objects have fringe reading reproducibility or less, indicating that the observed interference fringe component is represented only by the defocus term (W ₂ ).

Therefore, [n _t0 −n _m ] is obtained from the defocus term W _{2 of} the read fringe W ₀ (x, y), and Δn _t is obtained from the aberration term W.
(X, y) is obtained, and the two parameters can be separated. That is, (Se ₁ and Se ₂ are the maximum Sag amounts on both sides of the test object, as shown in Fig. 11.) Therefore, the refractive index distribution of the test object is calculated from the aberration term of the polynomial, and the refractive index of the matching liquid is calculated. If the index is known, the average refractive index of the test object can be obtained from the defocus term of the polynomial.

The refractive index n _m of the matching fluid may be obtained from the known refractive index and shape of the sample.

[Example] FIG. 1 shows a measuring apparatus used in the present invention, and this apparatus is basically a Mach-Zehnder interferometer. Reference numeral 1 denotes a coherent light source that emits coherent light (wavelength λ), for example, a He-Ne laser light source. The light beam emitted from the coherent light source 1 is expanded by the beam expander lens 2,
The collimator lens 3 forms a parallel light beam. 4 is the first
The light flux reflected by the half mirror 4 is further reflected by the movable mirror 5 and transmitted through the transparent container 6. The movable mirror 5 can arbitrarily finely move (tilt) its angle.

The transparent container 6 is formed of glass without distortion.
Reference numeral 7 denotes an openable / closable lid. In the transparent container 6, the material of the test lens 10 is polymethyl methacrylate (acryl).
In the case of (1), for example, a matching liquid 8 in which dimethyl silicone oil and phenylmethyl silicone oil are mixed is contained.

In the matching liquid 8, as shown in FIG.
A glass sample 9 having a known refractive index ng and a shape (θ, L) and a test lens 10 having a known refractive index n _t and a known shape (d, Sag (x, y)) are transmitted in parallel. It is arranged perpendicular to the light flux. The lens 10 to be inspected is generally a plastic lens such as polymethylmethacrylate (acrylic), but may be a lens made of glass or other material.

The light beam transmitted through the first half mirror 4 is reflected by the first fixed mirror 11 and then further reflected by the second half mirror 12.
Are superimposed on the light wave which has been reflected by the lens 10 and passed through the lens 10 to be inspected or the glass sample 9. Then, an interference fringe generated by the overlap of the two light waves forms an image on the surface of the imaging element 14 by the first imaging lens 13.

The first imaging lens 13 and the imaging element 14 are provided so as to be integrally movable in the vertical direction in the figure. Therefore, in FIG. 1, the first imaging lens
Although the lens 13 faces the lens 10 to be measured, the first imaging lens 13 may face the glass sample 9 as shown in FIG.

Returning to FIG. 1, the imaging element 14 is formed of, for example, 50 × 50 independent photoelectric elements. An AD converter 15, a digital operation circuit 16, and a display device 17 are sequentially connected to the output terminal. Further, the arithmetic circuit 16 is sequentially connected with a memory 18 for storing data necessary for the arithmetic operation and an input circuit 19 for inputting known data. Note that the arithmetic circuit 16 can be a microcomputer or other arithmetic device.

Reference numeral 20 denotes an observation device for observing the state in the transparent container 6, and includes a second fixed mirror 21 provided to face the second half mirror 12, a second imaging lens 22, It is composed of an imaging device 23 and a television monitor 24 provided at the image forming position.

In order to measure the refractive index of the lens under test 10 by the measuring method of the present invention, first, known data is input from the input circuit 19. Then, known data (n
g, θ, L) is input and stored in the memory 18. From the known data (d, Sag (x, y)) about the shape of the lens 10 to be tested, the maximum amount of Sag (Se ₁ + Se ₂ ) on both sides of the lens 10 to be tested and the wall thickness [d−Sag (x , y)] is calculated by the arithmetic circuit 16 and stored in the memory 18.

Next, the refractive index n _m of the matching liquid 8 is adjusted to a value slightly different from the refractive index n _{t of the} lens under test 10. This adjustment is performed while watching the television monitor 24 so that the number of interference fringes generated by the light wave that has passed through the lens 10 to be inspected is, for example, three or less. Specifically, with the lid 7 of the transparent container 6 removed, one of the two types of silicone oil forming the matching liquid 8 is dropped into the container 6 with a dropper or the like, and the matching liquid 8 is stirred and mixed. Good. When continuously measuring test lenses having the same design value, the matching liquid 8 may be adjusted only at the beginning.

Next, as shown in FIG.
Is made to face the glass sample 9. Then, as shown in FIG. 4, an interference fringe 30 generated by an overlap between the light wave transmitted through the glass sample 9 and the reference light wave forms an image on the image sensor 14. Then, based on the output from the image sensor 14 at that time, the arithmetic circuit 16 calculates the spatial frequency F of the stripe. This can be done with the well-known Discrete Fourier Transform (DFT), but faster Fast Fourier Transform (FFT)
Good.

In this case, from the output on the line segment 31 which is perpendicular to the interference fringes 30 on the image pickup device 14 as shown in FIG.
An intensity distribution of brightness in a substantially sinusoidal shape as shown in the figure is calculated. Then, from the intensity distribution, and calculating a spatial frequency F of the interference fringes by FFT, obtains the refractive index n _m of the matching liquid 8 from that value.

That is, since F = k / L (n _m −ng) L · tan θ = kλ, it can be obtained by n _m = ng + λ · F / tan θ. Then, the value of _nm is stored in the memory 18.

Next, as shown in FIG. 1, the first imaging lens 13
Is opposed to the lens 10 to be inspected. Then, the interference fringes 40 generated by the superposition of the light wave that has passed through the lens 10 to be inspected and the reference light wave form an image on the image sensor 14, as shown in FIG. The output from the image sensor 14 at that time is calculated by the arithmetic circuit 16
First, a high-precision fringe analysis is performed. This analysis is performed by, for example, a known spatial fringe scanning method by turning the movable mirror 5 by a slight angle to give a tilt to the stripe. Then, the phase is determined and subsequently expanded into a polynomial in the arithmetic circuit 16. In the present embodiment, it is expanded to Zernike polynomials, for example.

Then, the refractive index n _m of the matching liquid 8 and the maximum Sag amount (Se ₁ + Se ₂ ) are read from the memory 18, and the average refractive index n _t0 of the lens under test 10 is calculated from the defocus term of the polynomial. And outputs the result to the display device 17 for display.

Further, the thickness [d-Sag (x, y)] of each portion of the lens 10 to be inspected is read from the memory 18 and the aberration term (W) of the polynomial is read.
From, the refractive index distribution Δn _t (x, y) of the lens 10 under test is And outputs the result to the display device 17 for display.

(Measurement Example 1) As shown in FIG. 8, the refractive index and the refractive index distribution of a convex lens made of glass (BK9) having a thickness of 4.6 mm and a radius of 4 mm on each side were measured. Λ = 632.8nm measured by another method
The refractive index of BK9 for light of wavelength is 1.49255, and it is considered that the distribution is almost completely uniform.

The maximum Sag amount of 1.2 mm (0.6 × 2) and the number of observed fringes of about 1.5λ were observed, and the average refractive index was 1.4925 and the maximum refractive index distribution value was 0.174 × 10 ^-4 .

From this result, it can be seen that the average refractive index is measured extremely accurately, the refractive index distribution is also very small, and the error is very small.

(Measurement Example 2) The test lens made of polymethylmethacrylate (acrylic) was measured by changing the mixing ratio of the matching liquid. As a result, (1) when the refractive index of the matching liquid is 1.4901, the refractive index of the tested lens is 1.4903, the maximum refractive index distribution value is 0.511 × 10 ^-4 , and (2) when the refractive index of the matching liquid is 1.4907, the refractive index of the tested lens is The refractive index distribution maximum value was 0.852 × 10 ^-4 , which shows that the measurement result hardly changed even when the mixing ratio of the matching liquid changed.

(Measurement Example 3) The temperature was changed between 19 ° C and 25 ° C, and the test lens made of polymethylmethacrylate (acrylic) was measured.

The results are shown in FIG. 9 which shows the average refractive index n _t and FIG. 10 which shows the refractive index distribution.

As a result, the measured temperature dependence of the refractive index of the lens to be tested and the matching liquid is the theoretical value of the lens material (acrylic) at the d line and the theoretical value of the matching liquid (value measured by Abbe's refractometer). On the other hand, the rate of temperature change was almost the same.

Further, it can be seen that the maximum value of the refractive index distribution has only a change of 1.4 × 10 ⁻⁵ and has almost no temperature dependence.

[Effects of the Invention] According to the refractive index measuring method of the present invention, since the test object is only immersed in the matching liquid, it is not necessary to destroy the test object during the measurement. Moreover, from the output of the intensity distribution of the interference fringes, the average refractive index of the test object can be obtained by extracting the defocus term of the polynomial that represents the interference fringes, and the refractive index distribution of the test object can be obtained from the aberration term of the polynomial. It has an excellent effect that it can be obtained quantitatively, and therefore automatic analysis can be easily performed.

[Brief description of the drawings]

1 to 3 are schematic diagrams showing an example of a measuring apparatus for performing the measurement according to the present invention, FIGS. 4 to 6 are schematic diagrams showing a procedure for obtaining the refractive index of a matching liquid, and FIG. FIG. 8 is a schematic diagram showing interference fringes generated on the surface of the image pickup device by the light wave transmitted through the lens to be inspected, FIG. 8 is a schematic diagram of the lens to be inspected used in the first measurement example, and FIGS. 3 is a diagram showing the results of the measurement example, FIGS. 11 and 12 are schematic diagrams illustrating the measurement principle of the present invention, and FIG. 13 is a diagram illustrating the measurement accuracy according to the measurement principle of the present invention. .

Claims (4)

(57) [Claims] 1. A sample having a known refractive index and a shape, and an object having an unknown refractive index and a known shape are immersed in a matching liquid having a refractive index slightly different from that of the object to be coherent thereto. The light is transmitted, the transmitted light wave is superimposed on the reference light wave to generate interference fringes, the intensity distribution of the interference fringes generated by the light wave transmitted through the sample is output, and the refractive index of the matching liquid is obtained from the output. Along with it, the intensity distribution of the interference fringes generated by the light wave that has passed through the object to be inspected is output, and from the output, the defocus term and the aberration term of the polynomial representing the interference fringes are separated, and the defocus term and A refractive index measuring method, wherein an average refractive index of the test object is determined from the refractive index of the matching liquid. 2. A sample having a known refractive index and shape, and an object having an unknown refractive index and a known shape are dipped in a matching liquid having a refractive index slightly different from that of the object to be coherent. The light is transmitted, the transmitted light wave is superimposed on the reference light wave to generate interference fringes, the intensity distribution of the interference fringes generated by the light wave transmitted through the sample is output, and the refractive index of the matching liquid is obtained from the output. Along with it, the intensity distribution of the interference fringes generated by the light wave that has passed through the test object is output, and from the output, the defocus term and the aberration term of the polynomial representing the interference fringes are separated, and from the aberration term the above A refractive index measuring method, characterized in that a refractive index distribution of a test object is obtained. 3. The refractive index measuring method according to claim 1, wherein the sample is glass. 4. The refractive index measuring method according to claim 1, 2 or 3, wherein the test object is a lens.